(1) Field of Invention
The present invention relates to a structure of an electrode mount used for a low-pressure mercury vapor discharge lamp, an arc tube having an electrode using the mount, a low-pressure mercury vapor discharge lamp and a compact self-ballasted fluorescent lamp having the arc tube, and a method of manufacturing the arc tube.
(2) Related Art
Today in an age of energy conservation, low-pressure mercury vapor discharge lamps, especially compact self-ballasted fluorescent lamps, have been increasingly disseminated in the field of lighting technology as an energy saving light source that replaces incandescent lamps with low efficiency.
A compact self-ballasted fluorescent lamp includes an arc tube, a holder retaining the arc tube, an electronic ballast used for causing the arc tube to light and mounted on the holder, a case attached to the holder so as to house the electronic ballast therein, and a globe covering the arc tube and fixed to the case. Note that some compact self-ballasted fluorescent lamps include no globe (so-called “D” type).
The above-mentioned arc tube is formed by a glass tube, and includes electrodes fixed to both end portions of the glass tube. Each electrode includes a filament coil filled with an electron emissive material, a pair of lead wires supporting the filament coil therebetween in a manner that the filament coil hangs across the paired lead wires, and a glass member retaining the paired lead wire. Note that in this document, the word “electrode” is used to describe an electrode assembly being fixed to the end portion of the glass tube, and the electrode assembly having the same structure but having yet to be fixed to the end portion of the glass tube is referred to as a “mount”.
The glass member is, for example, bead glass (an electrode with bead glass is a so-called “bead-glass type”) as shown in FIG. 2 of Japanese Laid-Open Patent Application No. H01-163958, or a flare (an electrode with a flare is a so-called “flare type”) as shown in FIG. 3 of Japanese Laid-Open Patent Application No. H01-163958.
Along with a reduction in a diameter of the glass tube in response to the recent demand for making the arc tube compact, bead-glass type electrodes are widely used because they have a simple structure and facilitate the downsizing of the arc tube.
The process of fixing a bead-glass type mount to the end portion of the glass tube includes inserting the filament coil into the glass tube in a manner to dispose the bead glass inside the glass tube, and then crushing flatly the end portion of the glass tube that corresponds to a part of the paired lead wires, extending on the opposite side from the filament coil in relation to the bead glass.
In such a bead-glass type mount, the filament coil is supported by thin lead wires in a manner that it hangs across the lead wires at their tips, and therefore the filament coil would tend to wobble without the bead glass. The bead glass is thus used to provide a firm structure, preventing the filament coil from wobbling, and herewith it is possible to efficiently fix the mount (electrode) to the end portion of the glass tube.
On the other hand, electrodes (mounts) having no such a glass member have also been proposed (see FIG. 1 of Japanese Laid-Open Patent Application No. H01-163958). With an electrode (mount) having no glass member, the part thereof inserted into the glass tube can be made short. That is, it is possible to shorten the distance between the filament coil and the end portion of the glass tube, thereby allowing a size reduction of the arc tube.
There has also been a demand for cost and size reduction of arc tubes. In terms of the above electrode (mount) having no glass member, however, although a size reduction of the arc tube can be realized, it is sometimes the case that the filament coil comes detached from the lead wires as a result of, for example, an increase in the gap between the paired lead wires. Thus, the problem still remains that the process of fixing the mount to the end portion of the glass tube cannot be efficiently performed due to the difficulties in the handling.